In the operation of nuclear reactors, the nuclear energy source is in the form of hollow zircaloy tubes filled with enriched uranium, typically referred to as fuel assemblies. When the energy in the fuel assembly has been depleted to a certain level, the assembly is removed from the nuclear reactor. At this time, fuel assemblies emit both considerable heat and extremely dangerous neutron and gamma photons (i.e., neutron and gamma radiation). Thus, great caution must be taken when the fuel assemblies are handled, transported, packaged and stored. To protect the environment from radiation exposure, spent nuclear fuel is both transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel between locations while a storage cask is used to store spent nuclear fuel for a determined period of time.
Casks are typically designed to shield the environment from the dangerous radiation in two ways. First, shielding of gamma radiation requires large amounts of mass. Gamma rays are best absorbed by materials with a high atomic number and a high density, such as concrete, lead, and steel. The greater the density and thickness of the blocking material, the better the absorption/shielding of the gamma radiation. Second, shielding of neutron radiation requires a large mass of hydrogen-rich material. One such material is water, which can be further combined with boron for a more efficient absorption of neutron radiation.
The transfer cask must perform the vital function of providing adequate radiation shielding for both neutron and gamma radiation emitted by the enclosed spent nuclear fuel. The transfer cask must also be designed to provide adequate heat transfer. Guided by the shielding principles discussed above, transfer casks are made of lead or a gamma absorbing material and contain a neutron absorbing material as well. As stated previously, greater radiation shielding is provided by increased thickness and density of the gamma and neutron absorbing materials. The weight of a fully loaded transfer cask is typically in the range of 100-125 tons.
Similarly, storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because storage casks are not handled as much as transfer casks, the primary focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel. Size and weight are at best secondary considerations. As a result of maximizing the thickness of radiation absorbing materials, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh approximately 150 tons and have a height greater than 15 ft. A common problem is that storage casks cannot be lifted by nuclear power plant cranes because their weight exceed the rated capacity of the crane.
A common problem arises when the fully loaded transfer cask must be transported to the storage cask for the canister transfer procedure. Generally, the storage cask is located in a truck bay, or other location outside of the staging area. To get to the transfer cask, the storage cask may have to pass through a door of a nuclear plant's truck bay. The doors are typically 17-24 feet tall. The transfer casks are typically about 16 feet and 3 inches tall. The need to move casks into and out of enclosed facilities limits the size and shape of machines that can be used to move the casks. For example, a low ceiling in such a facility makes it impractical to use a boom or overhead crane to lift and transport casks. Similarly, a doorway not much larger than the cask itself limits the extent to which a lifting device can extend beyond the sides, top or bottom of the cask. Thus, a need exists for a low profile transporter that can withstand the weight of the storage cask.